Connected: An Internet Encyclopedia
RFC 950 - Appendix I. Address Mask ICMP

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### Appendix I. Address Mask ICMP

Address Mask Request or Address Mask Reply

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |     Type      |      Code     |          Checksum             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |           Identifier          |       Sequence Number         |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                        Address Mask                           |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          IP Fields:

             Addresses

                The address of the source in an address mask request message
                will be the destination of the address mask reply message.
                To form an address mask reply message, the source address of
                the request becomes the destination address of the reply,
                the source address of the reply is set to the replier's
                address, the type code changed to AM2, the address mask
                value inserted into the Address Mask field, and the checksum
                recomputed.  However, if the source address in the request
                message is zero, then the destination address for the reply
                message should denote a broadcast.

          ICMP Fields:

             Type

                AM1 for address mask request message

                AM2 for address mask reply message

             Code

                0 for address mask request message

                0 for address mask reply message

             Checksum

                The checksum is the 16-bit one's complement of the one's

                complement sum of the ICMP message starting with the ICMP
                Type.  For computing the checksum, the checksum field should
                be zero.  This checksum may be replaced in the future.

             Identifier

                An identifier to aid in matching requests and replies, may
                be zero.

             Sequence Number

                A sequence number to aid in matching requests and replies,
                may be zero.

             Address Mask

                A 32-bit mask.

          Description

             A gateway receiving an address mask request should return it
             with the address mask field set to the 32-bit mask of the bits
             identifying the subnet and network, for the subnet on which the
             request was received.

             If the requesting host does not know its own IP address, it may
             leave the source field zero; the reply should then be
             broadcast.  However, this approach should be avoided if at all
             possible, since it increases the superfluous broadcast load on
             the network.  Even when the replies are broadcast, since there
             is only one possible address mask for a subnet, there is no
             need to match requests with replies.  The "Identifier" and
             "Sequence Number" fields can be ignored.

                Type AM1 may be received from a gateway or a host.

                Type AM2 may be received from a gateway, or a host acting in
                lieu of a gateway.

### Related Information

- AM1 and AM2 defined - Appendix IV. Assigned Numbers
- RFC 792 - ICMP Protocol Specification

Next: Appendix II. Examples

Connected: An Internet Encyclopedia
RFC 950 - Appendix I. Address Mask ICMP